There are between 50,000 and 100,000 small water bodies in eastern and southern Africa and most of them are man made. The fishery potential of these small water bodies is not known and models to estimate their potential do not exist (Haight 1994).
For decades, detailed models have been developed to manage fish stocks in large water bodies. Two main approaches exist to manage fish stocks: the holistic and the analytical approach. While the former considers the fishery as a single unit of stock, the latter studies the factors that affect the expansion or the shrinkage of the population - the dynamics parameters. In the latter, stocks are separated into year classes or cohorts. Growth and mortality rates of these cohorts are described. Using growth and mortality rate estimators various models are applied to regulate fishing effort and/or fishing gears to obtain maximum sustainable yields. Over the years the analytical approach has gained more importance than the holistic approach because of the realisations of the complexity of fish stocks.
Methods to estimate growth and mortality rates have been developed for many fish species. To study growth various methods are used, for example, annular rings on scales and otoliths are sometimes used. This is possible because the deposition of calcium material on scales or otoliths changes with temperature. However, this method is difficult to use in the tropics due to low temperature fluctuations. Other methods to study growth include tagging individual fish and recapturing it at a specific time.
Tag recapture studies also used to estimate mortality rates as well as the number of fish in a water body. These methods are extensively reviewed by Jones (1977) and Ricker (1975). Growth and mortality rates can also be estimated by using length frequency distribution of the stock (Sparre 1989). In this method, the cohorts are described using modes of length frequency data.
To estimate fish production of the water resource, empirical models using morphometrics and limnology have been developed for lakes and other water bodies. Similar models have been developed for small water bodies in other parts of the world such as Thailand and Sri Lanka (Moreau and De Silva 1991).
Methods to estimate growth and mortality rates for fish in small water bodies in Africa do not exist. Existing empirical models to estimate yield of small water bodies using morphometrics, limnology or any other parameter which are used else where may not be applicable to small water bodies in Africa and in particular in the Southern Africa.
ALCOM GCP/RAF/277/BEL in collaboration with the Fisheries department of the Government of Malawi conducted a one-year study to test:
length-frequency and tag-recapture analysis as methods to estimate growth rate, mortality rate, production and yield of fish in small water bodies;
applicability of empirical models using limnological data to estimate fish production and yield in small water bodies.
